Apparatus and method of reporting logged measurement in wireless communication system

ABSTRACT

A method and apparatus of reporting logged measurements of an user equipment in a wireless communication system is provided. The user equipment in a Radio Resource Control (RRC) connected mode receives from a base station a Minimization of Drive Tests (MDT) configuration. The user equipment transitions from the RRC connected mode to an RRC idle mode and logs measurements based on the MDT configuration. The user equipment transmits to the base station a logging indicator indicating an availability of the logged measurement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisionalapplications 61/299,317 filed on Jan. 28, 2010, 61/323,320 filed on Apr.12, 2010, and Korean Patent Application No. 10-2011-0008067 filed onJan. 27, 2011, all of which are incorporated by reference in theirentirety herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for reporting loggedmeasurements in a wireless communication system.

2. Related Art

3rd generation partnership project (3GPP) long term evolution (LTE) isan improved version of a universal mobile telecommunication system(UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink, anduses single carrier-frequency division multiple access (SC-FDMA) in anuplink. The 3GPP LTE employs multiple input multiple output (MIMO)having up to four antennas. In recent years, there is an ongoingdiscussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPPLTE.

Minimization of driving tests (MDT) is a test performed by serviceproviders for coverage optimization by using a user equipment (UE)instead of using an automobile. A coverage varies depending on alocation of a base station (BS), deployment of buildings nearby, auser's usage environment, etc. Therefore, it is required for the serviceproviders to periodically perform the driving test, and a lot of costsand resources are consumed. The MDT is used when the service providermeasures the coverage by using the UE.

The MDT can be classified into an logged MDT and an immediate MDT.According to the logged MDT, after performing the MDT measurement, theUE delivers a logged measurement to a network available at a time of areporting condition. According to the immediate MDT, after performingthe MDT measurement, the UE delivers the measurement to the network atpoints in time when a configured reporting condition is satisfied. Thelogged MDT performs the MDT measurement in a radio resource control(RRC) idle mode, but the immediate MDT performs the MDT measurement inan RRC connected mode.

The logged measurement is a result of the logged MDT measurement, andcan be considered as data which is practically unnecessary to the UE.Accordingly, there is a need for a method capable of reporting thelogged measurement from the UE to the network without having an effecton an available memory and service quality.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for reportinglogged measurements in a wireless communication system.

The present invention also provides a method and apparatus for ahandover to report logged measurements in a wireless communicationsystem.

In an aspect, a method of reporting logged measurements of a userequipment in a wireless communication system is provided. The methodincludes receiving, by a user equipment in a Radio Resource Control(RRC) connected mode from a base station, a Minimization of Drive Tests(MDT) configuration, transitioning from the RRC connected mode to an RRCidle mode, logging, by the user equipment in the RRC idle mode,measurements based on the MDT configuration, and transmitting, by theuser equipment to the base station, a logging indicator indicating anavailability of the logged measurement.

The method may include transitioning from the RRC idle mode to the RRCconnected mode, and the logging indicator may be transmitted by the userequipment in the RRC connected mode.

The logging indicator may be included in a RRC Connection Setup Completemessage used to confirm a successful completion of an RRC connectionestablishment.

The logging indicator may be included in an RRC ConnectionReconfiguration Complete message used to confirm a successful completionof an RRC connection reconfiguration.

The MDT configuration may include a logging interval indicatingperiodicity for storing measurement results.

The method may further include receiving, by the user equipment from thebase station, an information request to request the logged measurements,and transmitting, by the user equipment to the base station, aninformation response to send the logged measurements.

In another aspect, an apparatus of reporting logged measurements in awireless communication system is provided. The apparatus includes aprocessor configured to log, in a Radio Resource Control (RRC) idlemode, measurements based on a Minimization of Drive Tests (MDT)configuration, and an interface unit configured to receive, in a RRCconnected mode, the MDT configuration and transmit a logging indicatorindicating an availability of the logged measurement.

Overhead caused by additional signaling for performing a minimizationdriving test (MDT) can be reduced. In addition, a logged measurement canbe received at a time desired by a network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a radio protocol architecture for a userplane.

FIG. 3 is a diagram showing a radio protocol architecture for a controlplane.

FIG. 4 is a flowchart showing a cell selection procedure of a userequipment (UE) in an idle mode.

FIG. 5 is a flowchart showing an RRC connection establishment procedure.

FIG. 6 is a flowchart showing an RRC connection reconfigurationprocedure. An RRC connection.

FIG. 7 is a flowchart showing a UE information reporting procedure.

FIG. 8 shows a procedure of performing an MDT.

FIG. 9 is a flowchart showing a method of reporting a logged measurementaccording to an embodiment of the present invention.

FIG. 10 is a flowchart showing a handover method according to anembodiment of the present invention.

FIG. 11 is a block diagram showing a wireless apparatus according to anembodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a radio protocol architecture for a userplane. FIG. 3 is a diagram showing a radio protocol architecture for acontrol plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Between different PHY layers, i.e., a PHY layer of a transmitter and aPHY layer of a receiver, data is transferred through the physicalchannel. The physical channel may be modulated using an orthogonalfrequency division multiplexing (OFDM) scheme, and may utilize time andfrequency as a radio resource.

Functions of the MAC layer include mapping between a logical channel anda transport channel and multiplexing/de-multiplexing on a transportblock provided to a physical channel over a transport channel of a MACservice data unit (SDU) belonging to the logical channel. The MAC layerprovides a service to a radio link control (RLC) layer through thelogical channel.

Functions of the RLC layer include RLC SDU concatenation, segmentation,and reassembly. To ensure a variety of quality of service (QoS) requiredby a radio bearer (RB), the RLC layer provides three operation modes,i.e., a transparent mode (TM), an unacknowledged mode (UM), and anacknowledged mode (AM). The AM RLC provides error correction by using anautomatic repeat request (ARQ).

Functions of a packet data convergence protocol (PDCP) layer in the userplane include user data delivery, header compression, and ciphering.Functions of a PDCP layer in the control plane include control-planedata delivery and ciphering/integrity protection.

A radio resource control (RRC) layer is defined only in the controlplane. The RRC layer serves to control the logical channel, thetransport channel, and the physical channel in association withconfiguration, reconfiguration and release of radio bearers (RBs). An RBis a logical path provided by the first layer (i.e., the PHY layer) andthe second layer (i.e., the MAC layer, the RLC layer, and the PDCPlayer) for data delivery between the UE and the network.

The setup of the RB implies a process for specifying a radio protocollayer and channel properties to provide a particular service and fordetermining respective detailed parameters and operations. The RB can beclassified into two types, i.e., a signaling RB (SRB) and a data RB(DRB). The SRB is used as a path for transmitting an RRC message in thecontrol plane. The DRB is used as a path for transmitting user data inthe user plane.

When an RRC connection is established between an RRC layer of the UE andan RRC layer of the network, the UE is in an RRC connected state, andotherwise the UE is in an RRC idle state.

Data is transmitted from the network to the UE through a downlinktransport channel. Examples of the downlink transport channel include abroadcast channel (BCH) for transmitting system information and adownlink-shared channel (SCH) for transmitting user traffic or controlmessages. The user traffic of downlink multicast or broadcast servicesor the control messages can be transmitted on the downlink-SCH or anadditional downlink multicast channel (MCH). Data is transmitted fromthe UE to the network through an uplink transport channel. Examples ofthe uplink transport channel include a random access channel (RACH) fortransmitting an initial control message and an uplink SCH fortransmitting user traffic or control messages.

Examples of logical channels belonging to a higher channel of thetransport channel and mapped onto the transport channels include abroadcast channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH), a multicasttraffic channel (MTCH), etc.

The physical channel includes several OFDM symbols in a time domain andseveral subcarriers in a frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. A resource block is aresource allocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Further, each subframe may use particularsubcarriers of particular OFDM symbols (e.g., a first OFDM symbol) of acorresponding subframe for a physical downlink control channel (PDCCH),i.e., an L1/L2 control channel. A transmission time interval (TTI) is aunit time of subframe transmission.

Hereinafter, an RRC state of a UE and an RRC connection mechanism willbe described.

The RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of an E-UTRAN. If the two layers are connectedto each other, it is called an RRC connected state, and if the twolayers are not connected to each other, it is called an RRC idle state.When in the RRC connected state, the UE has an RRC connection and thusthe E-UTRAN can recognize a presence of the UE in a cell unit.Accordingly, the UE can be effectively controlled. On the other hand,when in the RRC idle state, the UE cannot be recognized by the E-UTRAN,and is managed by a core network in a tracking area unit which is a unitof a wider area than a cell. That is, regarding the UE in the RRC idlestate, only a presence or absence of the UE is recognized in a wide areaunit. To get a typical mobile communication service such as voice ordata, a transition to the RRC connected state is necessary.

When a user initially powers on the UE, the UE first searches for aproper cell and thereafter stays in the RRC idle state in the cell. Onlywhen there is a need to establish an RRC connection, the UE staying inthe RRC idle state establishes the RRC connection with the E-UTRANthrough an RRC connection procedure and then transitions to the RRCconnected state. Examples of a case where the UE in the RRC idle stateneeds to establish the RRC connection are various, such as a case whereuplink data transmission is necessary due to telephony attempt of theuser or the like or a case where a response message is transmitted inresponse to a paging message received from the E-UTRAN.

A non-access stratum (NAS) layer belongs to an upper layer of the RRClayer and serves to perform session management, mobility management, orthe like.

To manage mobility of the UE in the NAS layer, two states are defined,i.e., an EPS mobility management-REGISTERED (EMM-REGISTERED) state andan EMM-DEREGISTERED state. These two states apply to the UE and the MME.Initially, the UE is in the EMM-DEREGISTERED state. To access a network,the UE performs a process of registering to the network through aninitial attach procedure. If the attach procedure is successfullyperformed, the UE and the MME enter the EMM-REGISTERED state.

To manage a signaling connection between the UE and the EPC, two statesare defined, i.e., an EPS connection management (ECM)-IDLE state and anECM-CONNECTED state. These two states apply to the UE and the MME. Whenthe UE in the ECM-IDLE state establishes an RRC connection with theE-UTRAN, the UE enters the ECM-CONNECTED state. When the MME in theECM-IDLE state establishes an S1 connection with the E-UTRAN, the MMEenters the ECM-CONNECTED state. When the UE is in the ECM-IDLE state,the E-UTRAN does not have context information of the UE. Therefore, theUE in the ECM-IDLE state performs a UE-based mobility related proceduresuch as cell selection or reselection without having to receive acommand of the network. On the other hand, when the UE is in theECM-CONNECTED state, mobility of the UE is managed by the command of thenetwork. If a location of the UE in the ECM-IDLE state becomes differentfrom a location known to the network, the UE reports the location of theUE to the network through a tracking area update procedure.

Next, system information will be described.

The system information includes essential information that must be knownto a UE to access a BS. Thus, the UE has to receive all of the systeminformation before accessing the BS. Further, the UE must always havethe latest system information.

Since the system information is information that must be known to allUEs in one cell, the BS periodically transmits the system information.

According to the section 5.2.2 of 3GPP TS 36.331 V8.4.0 (2008-12) “RadioResource Control (RRC); Protocol specification (Release 8)”, the systeminformation is classified into a master information block (MIB), ascheduled block (SB), and a system information block (SIB). The MIBallows the UE to know a physical configuration (e.g., bandwidth) of aparticular cell. The SB reports transmission information (e.g., atransmission period or the like) of SIBs. The SIB is a group of aplurality of pieces of system information related to each other. Forexample, an SIB includes only information of a neighbor cell, andanother SIB includes only information of an uplink radio channel used bythe UE.

In general, a service provided by the network to the UE can beclassified into three types to be described below. Further, according towhich service can be provided, the UE recognizes a cell typedifferently. A service type will be first described below, and then thecell type will be described.

1) Limited service: This service provides an emergency call and anearthquake and tsunami warning system (ETWS), and can be provided in anacceptable cell.

2) Normal service: This service denotes a public use service for generaluse, and can be provided in a suitable or normal cell.

3) Operator service: This service denotes a service for a networkservice provider, and a cell can be used only by the network serviceprovider and cannot be used by a normal user.

The service type provided by a cell can be classified as follows.

1) Acceptable cell: This cell serves a UE with a limited service. Thiscell is not barred from the perspective of the UE, and satisfies a cellselection criterion of the UE.

2) Suitable cell: This cell serves a UE with a regular service. Thiscell satisfies a condition of the acceptable cell, and also satisfiesadditional conditions. Regarding the additional conditions, this cellhas to belong to a PLMN to which the UE can access, and a tracking areaupdate procedure of the UE must not be barred in this cell. If thecorresponding cell is a CSG cell, this cell must be accessible by the UEas a CSG member.

3) Barred cell: Information indicating that a cell is a barred cell isbroadcast in this cell by using the system information.

4) Reserved cell: Information indicating that a cell is a reserved cellis broadcast in this cell by using the system information.

FIG. 4 is a flowchart showing a cell selection procedure of a UE in anidle mode.

The UE selects a public land mobile network (PLMN) and a radio accesstechnology (RAT) to receive a service (step S410). The PLMN and the RATmay be selected by a user of the UE, and data stored in a universalsubscriber identity module (USIM) may also be used.

Among cells of which signal strength or quality measured from a BS isgreater than a particular value, the UE selects a cell having a greatestvalue (step S420). Then, the UE receives system information periodicallysent by the BS. The particular value is a value defined in a system toguarantee quality of a physical signal in data transmission/reception.Accordingly, the value may vary depending on the RAT in use.

If network registration is required, the UE registers its owninformation (for example, IMSI) for receiving a service (for example,paging) from a network (steps S430 and S440). The network registrationis not performed whenever the UE selects a cell. For example, thenetwork registration is performed when system information (for example,Tracking Area Identity (TAI)) of the network to be registered isdifferent from network information known to the UE.

If a value of the signal strength or quality measured from the BS whichprovides a service to the UE is less than a value measured from a BS ina neighboring cell, the UE selects one of other cells providing a bettersignal property than that of a cell of the BS currently accessed by theUE (step S450). This process is referred to as cell reselection todistinguish it from initial cell selection of the step S420. In thiscase, the cell reselection may occur frequently according to changes inthe signal property, and to prevent this, time constraints may be given.

Next, a procedure for selecting a cell by the UE will be described indetail.

If the UE is turned on or is camped on a cell, the UE may performprocedures for selecting/reselecting a cell having suitable quality inorder to receive a service.

The UE in an RRC idle state needs to select the cell having suitablequality all the time, and thus be prepared to receive the servicethrough the cell. For example, the UE that has been just turned on mustselect the cell having suitable quality so as to be registered into anetwork. If the UE that has stayed in an RRC connected state enters intoan RRC idle state, the UE must select a cell on which the UE itself iscamped. In this manner, a process of selecting a cell satisfying acertain condition by the UE in order to stay in a service waiting statesuch as the RRC idle state is called cell selection. The cell selectionis performed in a state that the UE does not currently determine a cellon which the UE itself is camped in the RRC idle state, and thus it isvery important to select the cell as quickly as possible. Therefore, ifa cell provides radio signal quality greater than or equal to apredetermined level, the cell may be selected in the cell selectionprocess of the UE even though the cell is not a cell providing bestradio signal quality.

Hereinafter, by referring to the 3GPP TS 36.304 V8.3.0 (2008-09) “UserEquipment (UE) procedures in idle mode (Release 8)”, a method andprocedure for selecting a cell by a UE will be described in detail.

If power is initially turned on, the UE searches for available PLMNs andselects a suitable PLMN to receive a service. Subsequently, the UEselects a cell having a signal quality and property capable of receivinga suitable service among the cells provided by the selected PLMN.

The cell selection process can be classified into two processes.

One process is an initial cell selection process, and in this process,the UE does not have previous information on radio channels. Therefore,the UE searches for all radio channels to find a suitable cell. In eachchannel, the UE searches for the strongest cell. Subsequently, if asuitable cell satisfying cell selection criteria is found, the UEselects the cell.

The other process is a cell selection process using stored information,and in this process, the UE uses radio channel information stored in theUE, or selects a cell by using information being broadcasted from thecell. Accordingly, a cell may be quickly selected compared to theinitial cell selection process. If a cell satisfying the cell selectioncriteria is found, the UE selects the cell. If the cell satisfying thecell selection criteria is not found, the UE performs the initial cellselection process.

The cell selection criteria used by the UE in the cell selection processmay be represented by Equation 1 as shown:

Srxlev>0  [Equation 1]

where Srxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−Pcompensation.Qrxlevmeas denotes a measured cell received level (i.e., referencesignal received power (RSRP)), Qrxlevmin denotes a minimum requiredreceived level (dBm) in the cell, Qrxlevminoffset denotes a offset toQrxlevmin, Pcompensation is max(PEMAX−PUMAX, 0) (dB), PEMAX denotesmaximum transmission power (dBm) allowed for the UE in the correspondingcell, and PUMAX denotes maximum transmission power (dBm) for a radiofrequency (RF) transmission unit of the UE and based on performance ofthe UE.

In the above Equation 1, it can be seen that the UE selects a cellhaving signal strength and quality greater than a particular valuespecified in the cell providing the service. Further, the parametersused in the above Equation 1 are broadcast by using the systeminformation, and the UE receives those parameter values to use them forthe cell selection criteria.

If the UE selects a cell satisfying the cell selection criteria, the UEreceives information required for an RRC idle mode operation of the UEin the corresponding cell from the system information of thecorresponding cell. The UE receives all the information required for theRRC idle mode operation, and then waits in an idle mode to request aservice (for example, originating call) to a network or receive aservice (for example, terminating call) from the network.

After the UE selects a certain cell through a cell selection process,the signal strength and quality between the UE and the BS may be changeddue to the change of the UE mobility and wireless environment.Therefore, if the quality of the selected cell deteriorates, the UE mayselect another cell providing better quality. If a cell is reselected inthis manner, a cell providing signal quality better than that of thecurrently selected cell is selected in general. This process is calledcell reselection. A basic object of the cell reselection process isgenerally to select a cell providing best quality to the UE from theperspective of the radio signal quality.

In addition to the perspective of the radio signal quality, the networkmay notify the UE of a priority determined for each frequency. The UEthat has received the priority may consider this priority in the firstplace than the radio signal quality criteria during the cell reselectionprocess.

As described above, there is a method of selecting or reselecting a cellbased on the signal property of the wireless environment. When a cell isselected for reselection in the cell reselection process, there may becell reselection methods as described below, based on the RAT andfrequency characteristics of the cell.

-   -   Intra-frequency cell reselection: A reselected cell is a cell        having the same center-frequency and the same RAT as those used        in a cell on which the UE is currently being camped.    -   Inter-frequency cell reselection: A reselected cell is a cell        having the same RAT and a different center-frequency with        respect to those used in the cell on which the UE is currently        being camped.    -   Inter-RAT cell reselection: A reselected cell is a cell using a        different RAT from a RAT used in the cell on which the UE is        currently being camped.

The principles of the cell reselection process are as follows.

First, the UE measures quality of a serving cell and a neighboring cellfor cell reselection.

Second, the cell reselection is performed based on cell reselectioncriteria. The cell reselection criteria have following characteristicswith regard to the measurement of serving cells and neighboring cells.

The intra-frequency cell reselection is basically based on ranking. Theranking is an operation for defining a criterion value for evaluation ofcell reselection and for ordering cells according to a magnitude of thecriterion value by using the criterion value. A cell having the highestcriterion is referred to as a best-ranked cell. The cell criterion valueis a value to which a frequency offset or a cell offset is optionallyapplied on the basis of a value measured by the UE for a correspondingcell.

The inter-frequency cell reselection is based on a frequency priorityprovided by the network. The UE attempts to camp on at a frequencyhaving a top priority. The network may provide the same frequencypriority to be commonly applied to UEs in a cell by using broadcastsignaling or may provide a frequency-specific priority to each UE byusing dedicated signaling for each UE.

For the inter-frequency cell reselection, the network may provideparameters (e.g., frequency-specific offsets) for use in cellreselection to the UE for each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, the network may provide a neighboring cell list (NCL) foruse in the cell reselection to the UE. The NCL includes cell-specificparameters (e.g. cell-specific offsets) used in the cell reselection.

For the intra-frequency or inter-frequency cell reselection, the networkmay provide the UE with a black list, i.e., a list of cells not to beselected in the cell reselection. The UE does not perform the cellreselection on cells included in the black list.

Now, the ranking used in a cell reselection evaluation process will bedescribed.

A ranking criterion used to assign a priority to a cell is defined byEquation 2 as shown:

Rs=Qmeas,s+Qhyst, Rn=Qmeas,n−Qoffset  [Equation 2]

where Rs denotes a ranking value of a serving cell, Rn denotes a rankingcriterion of a neighboring cell, Qmeas,s denotes a quality valuemeasured for the serving cell by the UE, Qmeas,n denotes a quality valuemeasured for the neighboring cell by the UE, Qhyst denotes a hysteresisvalue for ranking, and Qoffset denotes an offset between two cells.

In the intra-frequency cell reselection, if the UE receives an offsetQoffsets,n between the serving cell and the neighboring cell,Qffoset=Qoffsets,n. Otherwise, Qffoset=0.

In the inter-frequency cell reselection, if the UE receives the offsetQoffsets,n, Qoffset=Qoffsets, n+Qfrequency. Otherwise,Qoffset=Qfrequency.

If the ranking criterion Rs of the serving cell and the rangingcriterion Rn of the neighboring cell are not much different from eachother and constantly vary, ranking orders of the serving cell and theneighboring cell may change frequently. Thus, the serving cell and theneighboring cell may be reselected alternately while changing theirranking orders too often. In order to prevent the UE from reselectingtwo cells alternately, the hysteresis value Qhyst is used to give ahysteresis in the cell reselection.

The UE measures the ranking criterion Rs of the serving cell and theranking criterion Rn of the neighboring cell according to the aboveequation. A cell having the greatest ranking criterion value isreselected by considering this cell as a best-ranked cell.

In the above-mentioned cell reselection criterion, the quality of cellsis considered as most important factor when performing the cellreselection. If a reselected cell is not a suitable cell, the UEexcludes the reselected cell or a frequency of the reselected cell fromtargets of the cell reselection.

FIG. 5 is a flowchart showing an RRC connection establishment procedure.

A UE sends to a network an RRC connection request message for requestingan RRC connection (step S510). The network sends an RRC connection setupmessage in response to the RRC connection request (step S520). Afterreceiving the RRC connection setup message, the UE enters an RRCconnected mode.

The UE sends to the network an RRC connection setup complete messageused to confirm successful completion of the RRC connectionestablishment (step S530).

A RRC connection reestablishment is similarly performed as the RRCconnection establishment. The RRC connection establishment is tore-establish the RRC connection, which involves the resumption of SRB1operation, the re-activation of security and the configuration of onlythe primary cell. A UE sends to a network an RRC connectionreestablishment request message for requesting an RRC connectionestablishment. The network sends an RRC connection reestablishmentmessage in response to the RRC connection reestablishment request. TheUE sends to the network an RRC connection reestablishment completemessage as a response for the RRC connection reestablishment.

FIG. 6 is a flowchart showing an RRC connection reconfigurationprocedure. An RRC connection reconfiguration is used to modify an RRCconnection. This is used to establish/modify/release an RB, to perform ahandover, to setup/modify/release measurements, and toadd/modify/release secondary cells.

A network sends to a UE an RRC connection reconfiguration message formodifying the RRC connection (step S610). In response to the RRCconnection reconfiguration, the UE sends to the network an RRCconnection reconfiguration complete message used to confirm successfulcompletion of the RRC connection reconfiguration (step S620).

FIG. 7 is a flowchart showing a UE information reporting procedure.

A network sends to a UE a UE information request message for obtainingUE information (step S710). The UE information request message includesa field for indicating whether the UE will report information on arandom access process and/or a radio link failure. The UE informationrequest message includes a field for indicating whether the UE willreport a logged measurement.

The UE sends to the network a UE information response message includinginformation requested by the UE information request (step S720).

Now, minimization of driving tests (MDT) will be described.

The MDT is a test performed by service providers for coverageoptimization by using a UE instead of using an automobile. A coveragevaries depending on a location of a BS, deployment of buildings nearby,a user's usage environment, etc. Therefore, it is required for theservice providers to periodically perform driving tests, and a lot ofcosts and resources are consumed. The MDT is used when the serviceprovider measures the coverage by using the UE.

The MDT can be classified into a logged MDT and an immediate MDT.According to the logged MDT, after performing the MDT measurement, theUE delivers a logged measurement to a network available at a time of areporting condition. According to the immediate MDT, after performingthe MDT measurement, the UE delivers the measurement to the network atpoints in time when a configured reporting condition is satisfied. Thelogged MDT performs the MDT measurement in an RRC idle mode, but theimmediate MDT performs the MDT measurement in an RRC connected mode.

FIG. 8 shows a procedure of performing an MDT.

The MDT includes an MDT configuration 810, an MDT measurement 820, andan MDT report 830 which are performed in that order.

The MDT configuration can be transmitted from a network to a UE via alogged measurement configuration message which is an RRC message. The UEcan receive the MDT configuration in an RRC connected mode. Even if theUE transitions to an RRC idle mode, the MDT configuration is kept, andthus an MDT measurement result is also kept.

The MDT configuration may include at least one of a logging interval, alogging duration, a reference time, and an area configuration. Thelogging interval indicates a periodicity for storing a measurementresult. The logging interval indicates a timer value for loggingduration. The reference time is used by the UE to echo back thereference in a logged measurement report. The area configurationindicates an area for which the UE is requested to perform logging.

The UE performs the MDT measurement based on the MDT configuration. Forexample, the MDT measurement is performed at every logging interval.

A measurement value may be a value well-known to those ordinary skilledin the art, such as reference signal received power (RSRP), referencesignal received quality (RSRQ), received signal code power (RSCP), andEc/No.

The UE sends to the network a logged measurement in the RRC connectedmode. In the logged MDT, the UE logs the measurement in the RRC idlemode. Then, upon re-entering the RRC connected mode, the UE sends thelogged measurement to the network.

The logged measurement may include at least one of measurement resultsof available serving cell measurements, measurement results of availableneighbor cell measurements, time information, and location information.

For the MDT report, the UE information reporting procedure of FIG. 7 canbe used. The network sends to the UE an information request including afield that indicates a report of the logged measurement. The UE sends tothe network an information response including the logged measurement.

The UE stores the logged measurement, and then sends the loggedmeasurement upon receiving a report request of the network. However, thenetwork is unable to know whether the logged measurement is stored inthe UE. In particular, this may be problematic after handover. Assumethat the UE uses a first cell as a serving cell, and starts the loggedMDT by receiving the MDT configuration from the first cell. Thereafter,if the UE performs a handover to a second cell, the second cell isunable to know whether the logged measurement is stored in the UE. Tosolve this problem, the network may request the UE to periodicallyreport whether there is the logged measurement, which may result in theincrease of signaling overhead.

Since transmission of the logged measurement is not related to servicequality of the UE and requires large amount of message, it is effectivein general for the network to receive the logged measurement at a timewhen traffic is relatively light. If the network agrees that the UEreports the logged measurements randomly, the UE may be unable to reportthe logged measurements at the most suitable time. Accordingly, it isnot preferable to accept that the UE reports the logged measurementsrandomly, and it is required that the UE reports the logged measurementswhen the network is request.

It is difficult for the serving to know whether the UE has the loggedmeasurements. If signals are exchanged to recognize whether there is thelogged measurement between the network and the UE, times and radioresources for the signaling are required.

FIG. 9 is a flowchart showing a method of reporting a logged measurementaccording to an embodiment of the present invention.

A UE receives an MDT configuration from a network (step S910). In thiscase, the UE operates in an RRC connected mode in which an RRCconnection is established to a serving cell.

The UE transitions to an RRC idle mode (step S920). Then, the UE logsmeasurements on the basis of the MDT configuration (step S930).

The UE establishes or re-establishes an RRC connection to a BS and thusenters the RRC connected mode (step S940). As the UE transitions fromthe RRC idle mode to the RRC connected mode, the UE sends a loggingindicator to the network (step S945). The logging indicator may be a1-bit indicator indicating an availability of the logged measurement.The UE performs an MDT measurement in the idle mode, and reports to thenetwork whether there is the logged measurement while entering theconnection mode.

The UE can send the logging indicator to the network when the RRCconnection is established, when the RRC connection is reestablished, orwhen the RRC connection is reconfigured. For example, when the RRCconnection procedure of FIG. 5 is performed, the logging indicator maybe included in the RRC connection setup complete message. When the RRCconnection reconfiguration procedure of FIG. 6 is performed, the loggingindicator may be included in the RRC connection reconfiguration completemessage.

Alternatively, the logging indicator may be transmitted by beingincluded in a measurement report message. The measurement report messageincludes the logging indicator together with a conventional normalmeasurement result.

When the network knows that there is the logged measurement on the basisof the logging indicator, the network sends to the UE an informationrequest for requesting a report of the logged measurement (step S950).

The UE sends to the network an information response including the loggedmeasurement (step S960). The information response may include thelogging indicator which indicates the existence of the loggedmeasurement.

FIG. 10 is a flowchart showing a handover method according to anembodiment of the present invention.

A UE receives an MDT configuration from a source cell in an RRCconnected mode (step S1010). Then, the UE performs an MDT measurementafter transitioning to an RRC idle mode.

The UE receives a handover command that instructs a handover from thesource cell to a target cell (step S1020). An RRC connectionreconfiguration message may be used as the handover command.

After performing the handover to the target cell, the UE sends to thetarget cell a handover complete to confirm completion of the handover(step S1030). An RRC connection reconfiguration complete message may beused as the handover complete. The handover complete includes a loggingindicator indicating availability of the logged measurement.

Therefore, the target cell can know that the logged measurement isstored in the UE on the basis of the logging indicator without having toreceive an additional message from the source cell.

FIG. 11 is a block diagram showing a wireless apparatus according to anembodiment of the present invention. The apparatus implements operationsof the UE according to embodiments shown in FIG. 9 to FIG. 10.

A UE 50 includes a processor 51, a memory 52, and an interface unit 53.The processor 51 implements the proposed functions, processes, and/ormethods. The processor 51 measures a logged MDT on the basis of an MDTconfiguration, and sends a logging indicator according toestablishment/re-establishment/modification of an RRC connection. Thememory 52 stores a logged measurement. The interface unit 53 provides aradio interface with respect to a BS. The interface unit 53 receives theMDT configuration from the BS through a radio channel, and sends thelogging indicator to the BS. The interface unit 53 may receive aninformation request and transmit an information response.

The processor may include application-specific integrated circuit(ASIC), other chipset, logic circuit and/or data processing device. Thememory may include read-only memory (ROM), random access memory (RAM),flash memory, memory card, storage medium and/or other storage device.When the embodiments are implemented in software, the techniquesdescribed herein can be implemented with modules (e.g., procedures,functions, and so on) that perform the functions described herein. Themodules can be stored in memory and executed by processor. The memorycan be implemented within the processor or external to the processor inwhich case those can be communicatively coupled to the processor viavarious means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

1. A method of reporting logged measurements of a user equipment in a wireless communication system, the method comprising: receiving, by a user equipment in a Radio Resource Control (RRC) connected mode from a base station, a Minimization of Drive Tests (MDT) configuration; transitioning from the RRC connected mode to an RRC idle mode; logging, by the user equipment in the RRC idle mode, measurements based on the MDT configuration; and transmitting, by the user equipment to the base station, a logging indicator indicating an availability of the logged measurement.
 2. The method of claim 1, further comprising transitioning from the RRC idle mode to the RRC connected mode, wherein the logging indicator is transmitted by the user equipment in the RRC connected mode.
 3. The method of claim 2, wherein the logging indicator is included in a RRC Connection Setup Complete message used to confirm a successful completion of an RRC connection establishment.
 4. The method of claim 2, wherein the logging indicator is included in an RRC Connection Reconfiguration Complete message used to confirm a successful completion of an RRC connection reconfiguration.
 5. The method of claim 1, wherein the MDT configuration includes a logging interval indicating periodicity for storing measurement results.
 6. The method of claim 5, wherein the step of logging the measurements includes logging the measurements at every logging interval.
 7. The method of claim 1, further comprising: receiving, by the user equipment from the base station, an information request to request the logged measurements; and transmitting, by the user equipment to the base station, an information response to send the logged measurements.
 8. The method of claim 7, wherein the logged measurements includes measurement results of at least one serving cell and time information.
 9. The method of claim 1, wherein a size of the logging indicator is one bit.
 10. An apparatus of reporting logged measurements in a wireless communication system, the apparatus comprising: a processor configured to log, in a Radio Resource Control (RRC) idle mode, measurements based on a Minimization of Drive Tests (MDT) configuration; and an interface unit configured to receive, in a RRC connected mode, the MDT configuration and transmit a logging indicator indicating an availability of the logged measurement.
 11. The apparatus of claim 10, wherein the logging indicator is included in a RRC Connection Setup Complete message used to confirm a successful completion of an RRC connection establishment.
 12. The apparatus of claim 10, wherein the logging indicator is included in an RRC Connection Reconfiguration Complete message used to confirm a successful completion of an RRC connection reconfiguration.
 13. The apparatus of claim 10, wherein the interface unit is configured to receive, from a base station, an information request to request the logged measurements and transmit, to the base station, an information response to send the logged measurements.
 14. The apparatus of claim 13, wherein the logged measurements includes measurement results of at least one serving cell and time information.
 15. The apparatus of claim 10, wherein a size of the logging indicator is one bit. 